The present invention relates to process for making molded products containing cis-polybutadiene or other elastomers, and a monomer capable of cross-linking the elastomer into a three-dimensional network. More specifically, the cross-links which are capable of being produced are relatively long and flexible. Such molding compositions are eminently suitable for the production of molded golf balls, particularly unitary molded golf balls. Molding compositions of this general class, and golf balls which can be produced from them, are described in U.S. Pat. Nos. 3,313,545, issued Apr. 11, 1967, and 3,438,933, issued Apr. 15, 1969.
There are several advantages of homogeneous, unitary construction for a golf ball, in contrast to the wound balls of the earlier art. Unitary golf balls can be produced with a perfect center of gravity and thus have excellent aero-dynamic properties, superior roll, and trueness of flight. Such golf balls are highly resistant to cutting and are often indestructible in normal play. These balls will return to round even when severely distorted, and thus maintain their superior flight characteristics after extended use.
Homogeneous, unitary golf balls, may be manufactured with better quality then conventional wound balls.
As contrasted with the conventionally covered wound balls, unitary balls maintain their playing characteristics in hot and in cold weather, have an excellent shelf-life, and will not waterlog. If the paint on the ball becomes worn or damaged, the balls may be very readily reclaimed by removing or stripping off the old paint and repainting. By contrast, conventional covered wound balls seldom last long enough to allow repainting.
Such unitary balls may be molded in mating precision hemisphere molds or dies into which a cylindrical or other shaped slug of moldable material is placed, as described in U.S. Pat. Nos. 3,313,535 and 3,438,933, the disclosures of which are incorporated herein by reference. The slugs may be cut from the extrudate of a mixer-extruder. After placing of the slug, of sufficient size to fill the mold, the mold halves are closed, and heat and pressure are provided for enough time to cure the moldable elastomer. The moldable material comprises an elastomer, a cross-linking monomer, a filler and a cross-linking catalyst. The balls after discharge from the mold are buffed, surface treated and then painted and stamped in conventional manner.
Improved unitary molded golf balls are described in application Ser. No. 250,147, filed on June 21, 1972, now pending. The improved unitary golf balls utilize a specific sub-group of cross-linking monomers and can yield molded golf balls with better rebound than prior unitary golf balls, and with superior playing qualities, equal to or surpassing the highest quality wound golf balls available, and maintaining the advantages of unitary balls. The improved unitary balls were made with decreased amounts of filler, preferably less than about 30 phr (parts per 100 parts by weight of elastomer) of filler and more preferably less than about 20 phr of filler. These low filler additions are postulated to give higher rebound and superior distance to these improved golf balls.
However, golf balls made commercially by the normal procedures described in application Ser. No. 250,147 tend to be non-uniform in compression, and roundness, and consequently are difficult to buff and stamp in the standard production equipment. As described in application Ser. No. 250,147, these golf balls do possess superior distance qualities as compared to the standard unitary golf ball, but the improved golf balls have erratic flight patterns, sometimes veering to the left, sometimes to the right, and sometimes going straight.
While such balls are quite satisfactory, and do constitute an improvement over the standard unitary golf balls with respect to distance, rebound and click, it is an object of the invention to improve further the properties of such molded balls with respect to flight pattern, uniformity of compression, roundness, durability, and the like.